1. Field of the Invention
This invention relates to a device for carrying out DC lighting of a DC discharge lamp.
A DC discharge lamp employing the DC discharge lamp lighting device of the kind referred to can be utilized effectively as a display element of a large scale image display apparatus employed at exposition grounds, athletic and baseball stadiums, and the like.
2. Description of the Related Art
Japanese Patent Publication No. 51-22311 discloses a device which is arranged to heat a filament of a DC discharge lamp with a filament current source, to apply between the filament and an auxiliary electrode a fine-discharging DC source through a current limiting resistance element having a high resistance value, and to apply between the filament and an auxiliary electrode a fine-discharging DC source through a current limiting resistance element having a high resistance value, and to apply between the filament and an anode (control electrode) a main-discharging DC source through a current limiting resistance element for supplying a main-discharging lamp current. In that device, therefore, a fine discharge is made to occur between the filament and the auxiliary electrode with the fine-discharging DC source, and normal lighting is carried out by applying a main discharge starting voltage higher than a fine discharge lamp voltage across the filament and the anode by means of the main-discharging DC source. In such a DC discharge lamp lighting device including current limiting resistance elements for fine and main discharging, a source voltage about twice as large as the main discharging lamp voltage has been required for the main-discharging DC source when the lighting characteristics of the DC discharge lamp, in particular the lamp voltage, luminance fluctuation, thermal characteristics of the lamp voltage, and the like are taken into account, so the heat generation in particular of the current limiting resistance element for the main discharging becomes extremely large. As a result, there have arisen the problems that a large scale image display apparatus including a large number of DC discharge lamps has to be provided with a large and expensive heat radiating means, that a thermal control for maintaining a proper temperature for excellent light emission efficiency has been made difficult due to the large heat generation, and the like.
Japanese Patent Application Laid-Open Publication No. 61-15194 discloses a discharge lamp lighting device in which an inductance element is employed as a current limiting element so as to reduce the heat generation. According to that device, a high frequency inverter is provided for converting a DC source voltage into a high frequency voltage, and the output of the high frequency voltage through a transformer of this high frequency inverter is applied through a choke coil and rectifying diode to a discharge lamp. Such conversion and application of the high frequency voltage generally require a complicated circuit arrangement, resulting in various problems such as increased manufacturing costs and the necessity of providing a measure against high frequency noise, which makes manufacture complicated. In order to light the DC discharge lamp stably even under a load fluctuation from a no load state to a full load state, it is necessary to keep the internal impedance of the high frequency inverter small. This causes the problem that the high frequency inverter must be a large size transformer so the entire lighting device becomes large and expensive.
U.S. Pat. No. 4,649,322 discloses a discharge lamp lighting device which allows a DC discharge lamp to be stably lit while reducing the heat radiation without increasing the size of the device. In that device, a filament in the DC discharge lamp is heated by a filament voltage source, a high voltage pulse is applied to the filament at predetermined intervals by a pulse generating means, and power is supplied from a DC power source to a control electrode of the discharge lamp for a power-supply maintenance time set by an instruction signal from an instruction signal generating means. According to that arrangement, stable lighting may be carried out by starting the lamp discharge with the high voltage pulses generated at the predetermined intervals, and luminous intensity regulation is realized by varying the power-supply maintenance time. However, the required provision of the high voltage pulse generating means as well as the DC power source including the instruction signal generating means is disadvantageous because it makes the related circuit arrangement complicated and makes the manufacturing costs high. In addition, the filament source voltage and high voltage pulses are applied constantly, so the consumption and the heat radiation cannot be adequately reduced.